In Thinking, Fast and Slow Daniel Kahneman says that effortful thinking—the kind that contributes to cognitive load—uses up glucose. So do physical activity and exerting self-control. To cope with our daily cognitive demands, we’ve evolved heuristics, mental shortcuts we use in our decision making to avoid taxing our working memory. These shortcuts let us process information automatically so that we don’t use up as much of our precious glucose reserves. (The problem is that sometimes these shortcuts lead us astray as cognitive biases.)
In other words, mental burden—stuff you have to worry about and actively think about—uses up energy, and evolutionarily it’s in our best interest to try to find ways to reduce it. If people don’t see a payoff in investing their resources into active thinking, they won’t bother.
In a way, imposing unproductive mental burden on someone against their will can be considered a type of mental assault: not only does it reduce their working memory capacity and hence their ability to learn and innovate, but it also reduces the amount of energy they can devote to other functions.